OBJECTIVE In order to demonstrate cortical hyperexitability and to determine the efficacy of 'SEP of high amplitude (HASEP) in detecting asymptomatic relatives of JME patients, we evaluated the EEG findings and SEP in asymptomatic siblings of JME patients. METHODS We analysed the EEG and SEP findings of 37 JME patients and 48 asymptomatic siblings of 27 patients and compared the results with healthy volunteers. RESULTS Of 13 asymptomatic siblings with EEG abnormalities (27.1%), 5 had 4-6 Hz spike/polyspike and wave paroxysms (10.4%), 5 had slow wave paroxysms (10.4%), and the remaining 3 patients exhibited focal spike and wave activity during hyperventilation (6.25%). No significant difference was found between JME patients, asymptomatic siblings and control groups with respect to N20 latencies. N20/P25 amplitudes were significantly higher in both JME group and asymptomatic siblings than those of control cases. HASEP were observed in 8 out of 37 JME patients (21.6%), 10 out of 48 healthy siblings (20.8%) and none of the control cases. The consanguinity rate was higher in parents of patients who had at least one asymptomatic sibling with EEG and/or SEP abnormalities (68.7%) than in those of patients who had siblings with no EEG and SEP abnormalities (9.9%). We suggest that the high rate of SEP and EEG changes among offspring of consanguineous marriages might reflect the genetic heterogeneity of the disease.